This invention relates to cutting elements used in earth boring bits for drilling earth formations. Specifically this invention relates to cutting elements having a non-planar interface including a plurality of shallow abutting depressions between their substrate and their cutting layer.
A typical cutting element is shown in FIG. 1. The cutting element typically has cylindrical cemented carbide substrate body 2 having an end face 3 (also referred to herein as an xe2x80x9cupper surfacexe2x80x9d or xe2x80x9cinterface surfacexe2x80x9d). An ultra hard material layer 4, such as polycrystalline diamond or polycrystalline cubic boron nitride, is bonded on to the upper surface forming a cutting layer. The cutting layer can have a flat or a curved upper surface 5.
Generally speaking the process for making a compact employs a body of cemented tungsten carbide where the tungsten carbide particles are cemented together with cobalt. The carbide body is placed adjacent to a layer of ultra hard material particles such as diamond of cubic boron nitride (CBN) particles and the combination is subjected to high temperature at a pressure where diamond or CBN is thermodynamically stable. This results in recrystallization and formation of a polycrystalline diamond or polycrystalline cubic boron nitride layer on the surface of the cemented tungsten carbide. This ultra hard material layer may include tungsten carbide particles and/or small amounts of cobalt. Cobalt promotes the formation of polycrystalline diamond or polycrystalline cubic boron nitride and if not present in the layer of diamond or CBN, cobalt will infiltrate from the cemented tungsten carbide substrate.
The problem with many cutting elements is the development of cracking, spalling, chipping and partial fracturing of the ultra hard material cutting layer at the layer""s region subjected to the highest impact loads during drilling especially during aggressive drilling. To overcome these problems, cutting elements have been formed having a non-planar substrate interface surface 3 which is defined by forming a plurality of spaced apart grooves or depressions that are relatively deep in that they typically have a depth that is greater than 10% of the cutting element diameter. Applicants have discovered that these deep grooves or depression cause the build-up of high residual stresses on the interface surface leading to premature interfacial delamination of the ultra hard material layer from the substrate. Delamination failures become more prominent as the thickness of the ultra hard material layer increases. However, the impact strength of the ultra hard material layer increases with an increase in the ultra hard material layer thickness.
Consequently, a cutting element is desired that can be used for aggressive drilling and which is not subject to early or premature failure, as for example by delamination of the ultra hard material layer from the substrate, and which has sufficient impact strength resulting in an increased operating life.
The present invention provides for cutting elements which are mounted in a bit body. An inventive cutting element has an increased thickness of the ultra hard material cutting layer at its critical edge, while at the same time having a reduced tendency for delamination of the ultra hard material layer from the substrate. The critical edge of the cutting element is the portion of the edge of the cutting layer that comes in contact with the earth formations during drilling and is subject to the highest impact loads.
The inventive cutting element substrate interface surface over which is formed the ultra hard material cutting layer comprises a plurality of abutting shallow depressions. These depressions preferably span at least 20% of the interface substrate surface and extend to the periphery of the substrate coincident with the critical edge. The depressions may span the entire interface surface.
In one embodiment, a cutting element of the present invention comprises an interface surface that may be flat, convex i.e., dome shaped, or concave. A plurality of abutting shallow depressions are formed on the interface surface such the each shallow depression shares at least one side with another depression. Preferably each depression abuts at least two other depressions, i.e., each depression shares one side with a second depression and another side with a third depression. The depressions are preferably shallow in that their maximum depth is not greater than 5% and not less than 0.5% of the diameter of the cutting element. Moreover, the maximum width of each depression is not greater than 40% and not less than 1% of the diameter of the cutting element. In a preferred embodiment, the shallow depressions are concave in cross-section. Furthermore, with the exception of the depressions intersecting the periphery of the substrate, the remaining depressions are polygonal in shape when viewed from an axial direction of the cutting element. In other words, the sides of the depressions defining the depression perimeters are linear when viewed from an axial direction of the cutting element.